NTSB reverses findings that pinned fatal accident on Tamarack winglets
Probable cause now undetermined
Previously attributed to a malfunction of Tamarack’s active winglet system, the NTSB now says that the probable cause of a 2018 crash of a Cessna Citation could not be determined based on the available evidence.
Tamarack Aerospace Group, based in Sandpoint, Idaho, created the Active Technology Load Alleviation System (ATLAS) that includes winglets to increase lift and aerodynamic efficiency, coupled with automated control surfaces similar to ailerons that are directed by a control unit to move in response to increased wing loads, and offset those loads. The system allows winglet retrofits without requiring additional structure to be added, along with weight. Those control surfaces were installed on dozens of aircraft by the time a Cessna Citation CJ2 on an IFR flight from Indiana to Chicago crashed after a loss of control in flight. The NTSB investigation determined, in 2021, that the ATLAS system had malfunctioned, inducing a roll from which the aircraft did not recover.
In a rare (if not unprecedented) reversal the board wrote that Tamarack's petition "is granted in part because the available evidence for this accident does not sufficiently show that the ATLAS was the cause of the in-flight upset from which the pilot was unable to recover. In addition, the factual and analysis sections of the report and the findings have been revised to reflect the information presented in the petition response sections addressing witness marks, [control unit] bent pins, and the UK uncommanded roll event."
The FAA grounded all ATLAS-equipped aircraft (91 at the time) in May 2019, while the investigation of the Indiana accident was ongoing, citing that investigation along with six reports of uncommanded roll events submitted to the Aviation Safety Reporting System, or to European regulators. Tamarack filed for bankruptcy protection soon after, though the company continued to produce and install active winglets, and emerged from bankruptcy in 2021.
Tamarack President Jacob Klinginsmith said in a telephone interview February 29, “When a petition for reconsideration is filed, it's elevated, you know initially this investigation was done by a regional branch of the FAA and the NTSB, but the petition elevates it to the national level and so a new investigator in charge is assigned and they look at it with fresh eyes and so that’s what happened here. So, we’re just very pleased that the process worked just as it’s supposed to here and we have high praise for the NTSB for looking at this objectively from an engineering perspective to make sure all the facts are correct.”
The NTSB's original report stated, “In summary, the circumstances of the accident are consistent with asymmetric or trailing edge up deployment of the left [Tamarack Active Control Surface] for reasons that could not be determined."
The revised final report reaches a different conclusion, though stops short of determining a probable cause: “In summary, the circumstances of the accident are consistent with a left roll that began for reasons that could not be determined based on the available evidence. Although the resultant roll rate was above the nominal threshold for detection by the human vestibular system, the roll rate likely went unrecognized by the pilot, due primarily to the pilot’s attention being directed toward a checklist and communications with a controller, a lack of visible horizon because the airplane was in the clouds, and the autopilot engagement. After the autopilot disconnected, the pilot was audibly surprised and did not reduce engine power or deploy the speed brakes. The pilot was not able to regain control before collision with terrain."
When asked about probable cause, Klinginsmith said, “The NTSB’s response to our petition stands on its own as far as what we’ve provided to them and how they’ve responded. So we’re happy to kind of bring a close to this extended investigation.”
“Our fleet has continued to grow and the customers that are flying this [Active Winglet upgrade] see the safety and performance benefits every time they fly.”
